In power shifting transmissions, a fluid operated friction torque transmitter, such as a clutch or brake, is actuated during a ratio interchange. When electronic controls are utilized, the engaging pressure is generally controlled by a solenoid operated valve. To reduce the cost, a simple pulse width modulated (PWM) solenoid is used. These devices control the pressure in response to the duty cycle imposed on the solenoid coil. The resulting pressure has an AC component of approximately 60 Hz and a DC component proportional to the main system pressure as determined by the duty cycle.
The AC component can induce pressure pulsations at the torque transmitter, such that a vibratory motion can result in noise or a transient vibration sensed by the operator during the ratio interchange. To alleviate this condition, it is common practice to position an accumulator of significant size downstream of the control valve in parallel flow relation with the torque transmitter. This requires additional space and sufficient fluid during the ratio interchange to fill the accumulator resulting in increased size of the transmission envelope and the fluid necessary to complete the shift.
When space availability is limited, one or more wave springs can be used in the torque transmitter engaging mechanism, or a more expensive proportional solenoid valve and control unit can be used. These solutions, while quite effective, result in increased cost of production.